Preflight CCH Fiber payments before creating LND invoices

[jjyr]

Summary

• run a side-effect-free Fiber payment preflight before creating an LND hold invoice in the CCH receive-BTC flow
• apply the same fee and TLC expiry limits to the preflight and the eventual outgoing payment
• support preflight requests through both in-process and RPC-backed Fiber agents
• add regression coverage for rejecting unroutable Fiber invoices before contacting LND

Motivation

CCH previously created an externally payable LND hold invoice before checking whether the outgoing Fiber invoice was currently routable. When route construction failed, the BTC-side payment could already be accepted even though the order could not complete. The new dry-run check rejects these requests before creating payable LND state and does not create a Fiber payment session or send a TLC.

Validation

• cargo fmt --all -- --check
• cargo nextest run -p fnn --features sqlite test_receive_btc_preflights_fiber_sendability_before_lnd
• cargo clippy --all-targets --features sqlite -p fnn -- -D warnings